1. Field of the Invention
The present invention relates to a color electrophotographic device and method and, in particular, relates to a color electrophotographic device and method which include a control suitable for producing color picture images on transparent plastic sheets which are to be used for overhead projectors.
2. Description of Related Art
A color electrophotographic device produces color picture images through electrophotographic processes fundamentally by making use of a yellow (Y) series toner, a magenta (M) series toner, a cyan (C) series toner and a black (K) toner (in some cases the K toner is not used).
Examples of color electrophotographic devices and methods proposed up to now are as follows;
JA-A-60-17461(1985) and JP-A-60-17462(1985) which correspond to U.S. Pat. No. 4,578,331 disclose a multi overlaid development device and method wherein a toner image forming process is carried out in a multi overlaid manner on a photo sensitive drum (a primary recording medium) by making use of a Y series toner, an M series toner, a C series toner and a K toner to form a color toner image and then the formed color toner image is inclusively transferred and fixed on a secondary recording medium.
Further, U.S. Pat. No. 4,652,115 discloses an intermediate image transfer device and method wherein a Y series toner image, an M series toner image, a C series toner image and a K toner image are intermediately transferred in a multi overlaid manner to an intermediate image transfer medium to form a color toner image and then the formed color toner image is finally and inclusively transferred and fixed on a secondary recording medium.
Still further, JP-A-55-38502(1980) discloses a multi overlaid image transfer device and method wherein a Y series toner image, an M series toner image, a C series toner image and a K toner image held on an image transfer drum are transferred sequentially at every one rotation thereof onto a secondary recording medium in a multi overlaid manner to form and fix a color toner image thereon.
Still further, JP-A-59-50460(1984) discloses a tandem drum device and method wherein dedicated photo sensitive drums are provided for respective color toner images, latent images on the respective dedicated photo sensitive drums are developed with the respective dedicated toners to form respective color toner images thereon, and the respective toner images are sequentially transferred onto a secondary recording medium to form and fix a color toner image thereon.
Color electrophotographic devices using the above devices and methods could produce a desirable color picture image on an ordinary paper serving as a secondary recording medium, but could not produce a desirable color picture image on a transparent plastic sheet serving as a secondary recording medium.
When a transparent plastic sheet is used as a secondary recording medium, it is necessary to heighten the transmission factor of non-scattering light passing through a toner image for a desirable projection, and for this reason it is necessary to provide a sufficiently large energy for fixing toner images onto the transparent plastic sheet during the fixing process in comparison with that for an ordinary paper. For providing a large fixing energy for the transparent plastic sheet, measures such as a second fixing means for a transparent sheet and a retransfer mechanism for a transparent sheet which causes the transparent sheet to pass twice through the same fixing means were proposed. However, these measures included problems such as lower reliability and high cost because of an increased number of parts. Another measure contemplated was a method of reducing a second recording medium transferring speed during the fixing process when the second recording medium is a transparent sheet, however this method necessitated that the physical distance between the image transfer station and the fixing station be more than the length of the transparent sheet to be used in order to compensate for the speed difference between the image transfer process and the fixing process and caused a problem of increasing the size of the device. A further measure contemplated was to reduce the processing speed not only at the fixing process but also at all of the processes, but with this method respective specifications for an ordinary paper and a transparent sheet such as for a charging process, exposure process and development process in an electrophotographic process had to be separately determined which caused problems of increasing the complexity of the control of the device therefor and prolonging the production time of a color toner image.